The fundamental physical mechanisms associated with separation control using harmonic and steady VGJs are investigated next. The \(\lambda _2\)  vortex identification [1]} provides insight into the underlying flow physics of controlled LSBs and nature of the vortical structures and their evolution. The \(\lambda _2\) -criterion is based on a decomposition of the velocity gradient tensor, \(\partial u_i/\partial x_j\) , into the symmetric part (the rate-of-strain-rate tensor, \(S_{ij}\) ) and the antisymmetric part (the vorticity tensor, \(\Omega _{ij}\) ). Consider the three real eigenvalues (\(\lambda _1\le \lambda _2\le \lambda _3\) ) of the symmetric tensor \(S_{ik} S_{kj}+\Omega _{ik} \Omega _{kj}\) , a vortex region is defined where \(\lambda _2<0\)  according to Jeong and Hussain [1]}. Figures REF  and REF  present the instantaneous iso-surfaces of the \(\lambda _2\) -criterion colored by streamwise velocity in top-down and perspective views for harmonic and steady VGJs, respectively. A close-up perspective view of the flow structures immediately downstream of the actuator is presented in Figures REF  and REF  (right plot) as well. These structures are associated with the high-amplitude forcing through the jet hole.
